The usual first step in the preparation of secondary fibre stock from wastepaper is ordinarily carried out in a device called a pulper. The wastepaper is placed, together with a quantity of water, in an open-topped container called a pulper vat or pulper tub. It is then subject to the action of an impeller or rotor, mounted on and revolving with a shaft. Vigorous agitation of the liquid is produced, and, partly due to the fluid turbulence and fluid shear, and partly due to mechanical impact and abrasion from the rotor, the pieces of wastepaper are separated into their component fibres.
Pulpers in general use in the industry can be considered as divided into two general groups, those with the rotor mounted on a vertical shaft, and those with the rotor mounted on a horizontal shaft.
In the vertical shaft type, the tub usually takes the form of an open-topped cylinder with a vertical axis, and having a generally horizontal bottom, the whole supported on a suitable frame. Although the circular, or cylindrical tub is most common, lobed tubs of more or less clover-leaf shape are in use, square tubs are known, and various geometrical shapes are possible. Although the bottom is generally flat, a large radius curved fairing may connect the bottom with the sides, or there may be a frustoconical section of the lower side wall, joining with a circular horizontal center section of the bottom.
The rotor shaft, supported on suitable bearings, and driven by a suitable motor and drive, enters the tub axially from the bottom, through a suitable shaft seal or stuffing box.
The rotor itself is equipped with blades or vanes, usually backward-curved, with a general appearance somewhat resembling the impeller of a centrifugal pump, although not equipped with the front or back shrouds common on pump impellers.
The action of the rotor causes a primary circulation of the water, stock, and as-yet-unpulped paper in the tub which is in the form of a strong vortex about the axis of the cylindrical tub. A secondary circulation, well known in hydrodynamics in vertical flow regimes, is induced. This secondary circulation flows upwardly near the tub walls, radially inward near the surface, downward in the center toward the center of the rotor, i.e. toward the eye of the impeller, in the centrifugal pump analogy, and outward toward the walls near and between the rotor vanes.
It is common to provide a plurality of fixed guide vanes, usually angled upward in the flow direction, fastened to the lower part of the tub wall. Their purpose is to augment and direct the secondary flow, so that there is a strong downward current toward the center of the rotor, with the intent of bringing as-yet-unpulped paper downward into contact with the rotor more quickly. It is also known to mount a corkscrew-like device to the top of the rotor, intended to pull unpulped paper down into contact with the rotor.
It is usual to provide extraction plates, by which is meant an area in the bottom of the tub, usually underneath the rotor, which is covered by plates having a plurality of rather small holes, whereby the processed pulp stock is withdrawn from the pulper tub. Passing through the extraction holes, the stock passes to a closed extraction chamber usually of annular shape beneath the tub bottom. From there it is withdrawn through a pipe with a valve, usually by means of a pump, and sent to further processing.
Pulpers with a rotor mounted on a horizontal shaft, often spoken of as side-entry pulpers, have the rotor revolving in a vertical plane at one end of the tub. The tub usually has the form of a cylinder with a horizontal axis, with a cut-out area at the top, usually surrounded by short vertical sides. The extraction plates are vertical, behind the rotor, and an extraction chamber behind that.
Pulpers are spoken of as being either batch pulpers or continuous pulpers, although this refers more to their operating cycle than it does to any specific features of design or construction, with the possible exception of the size of the holes in the extraction plates.
In batch operation, the pulper is filled to its operating level with water or whitewater, by which is meant recycled water from mill operations, usually from thickeners, which contains a small concentration of fibres and fines. Wastepaper is added, normally in the form of bales. The pulper is operated for some length of time, following which the valve in the pipe from the extraction chamber is opened, the pump is placed in operation, and stock is withdrawn from the pulper through the extraction plates. It is not usual to attempt to empty the pulper completely at this point, particularly as there may remain pieces of paper which are as yet unpulped, and would not pass through the holes in the extraction plates. The stock in the pulper is extracted until the level falls substantially, and then the valve is closed again. The pulper is then refilled to operating level with water or whitewater, and more baled wastepaper is added. It is usual to leave the pulper rotor rotating at all times, that is, during extraction and refilling etc.
In continuous operation, the extraction valve is left open, and the extraction pump left running continuously, or at least for a substantial part of the time. Water or whitewater is added continuously, or at short intervals, to maintain the operating level at all times. Wastepaper is added in smaller quantities at relatively short intervals, to maintain a relatively uniform consistency, or concentration of pulp fibres in water, at all times.
What has been stated about wastepaper also applies to waste cardboard, or waste corrugated containers, all of which are used as furnish for secondary fibre paper and paperboard making operations.
It may be remarked here that, in a vertical shaft pulper, the vortical circulation is strong enough that the liquid surface of the circulating stock is not level, but exhibits a deep depression in the center of the vortex.
The wastepaper as received by the mill contains undesirable contaminants. Some observers have estimated that wastepaper received by United States mills contains, on the average, 15% by weight of contaminants.
Elongated material, including lengths of wire, lengths of steel strapping, lengths of rope or cord, and pieces of fabric, can be removed readily by a ragger. Ragger design has become pretty well standardized in most mills. It consists simply of a rope of rather large diameter, suspended from a winch, with the winch mounted near and somewhat above the top of the vertical side of the pulper, and the free end of the rope trailing in the stock of the vortex. The elongated material eventually becomes tightly wound around the rope, due to the action of the vortex, and can be removed from the pulper using the winch.
Heavy contaminant material, including nuts, bolts, metal parts, stones and the like, is often called junk in the industry. It is usually removed from the pulper by devices variously called junkers, junk towers, or junk boots. There are a variety of designs which have been used or proposed. What they have in common is an opening in the lower part of the pulper tub sidewall, at the bottom or somewhat above the bottom. The opening is connected by a connecting sleeve, or a chute or a slide to a chamber external to the pulper tub, which, in the common junk tower type of design, is square or rectangular in horizontal cross-section, and which extends vertically from somewhat below the pulper bottom to somewhat above the top of the pulper side wall. It is closed at the bottom, and open to atmosphere at the top.
In operation, a piece of junk, under the action of the rotating rotor, or driven by the strong vortical fluid flow, will move in generally circular motion on or near the tub bottom. When it reaches the opening, it will pass through it due to centrifugal force, and come to rest on the bottom of the junk tower. From there, the accumulated junk can be removed by using a clam-shell or orange-peel grapple, suspended from an overhead electric hoist, which can be lowered into the junk tower with its jaws or claws opened, then closed around the junk, and hoisted out. If the suspending hoist is mounted on an overhead trolley or a swinging gantry, it can then be dumped into a removable bin or container, which can later be trucked away to disposal. Alternatively, bucket elevator type devices are used, having a plurality of buckets mounted on a pair of endless chains, running over sprockets from the bottom of the tower, where they pick up junk, to the top where they dump into a container. Other means for junk removal from the tower have been used.
There is no flow of stock through the opening into the junk tower. The liquid level or stock level in the top of the tower will be above the stock level in the pulper tub, since the velocity head of the flowing liquid in the vortex at the opening, is converted to increased static head of the motionless liquid at the top of the tower, in accordance with the Bernouilli equation. Couture, in U.S. Pat. No. 4,030,671, using somewhat different language, points this out in his "Background of the Invention" section, where he says: ". . . the normal operation of a pulper rotor tends to force sufficient liquid from the tub to the junk remover when the pulping operation commences to maintain a higher static head in the junk remover than in the tub, commonly of the order of 12 inches." (Col. 1, lines 34-38). Vokes, in U.S. Pat. No. 4,129,259 using very similar language to Couture, says, under "Background of the Invention": ". . . the normal operation of a pulper rotor tends to force sufficient liquid from the tub to the junk remover when the pulping operation commences to maintain a higher static head in the junk remover than in the tub, commonly of the order of two or more feet." (Col. 1, lines 33-36).
Light weight contaminants are a particular problem in the pulping of waste paper. These include pieces of wood and the like, but mainly include plastics of various types and chemical compositions, of various sizes and in sheet, film, or foam form. The proportion of plastic contained in wastepaper seems to be increasing every year. Plastic sheet, or film, in the pulper does not readily disintegrate, as does paper or paperboard. The result is that it tends to accumulate in the pulper tub, from batch to batch in batch pulping, or progressively as the pulping operation is continued, in continuous pulping. As this goes on, the pulper's ability to defiber paper is reduced, until the throughput through the extraction plate is only a fraction of what it had been. The pulper is then sometimes said to be "constipated." One possible remedy is to shut down, drain the pulper, and manually remove the accumulated plastic. This is expensive, and time consuming.
Some comments on styrofoam may be appropriate here. Styrofoam is very common in baled wastepaper. There would scarcely be a bale to be found without a few crushed styrofoam coffee cups. Also, larger chunks of styrofoam, such as the sort of shaped blocks used to pack cameras or instruments for shipment, are very common among wastepaper. Since styrofoam is very light weight, a block of styrofoam is very buoyant, and will float high in water or stock. It is not unusual to see a large block of styrofoam in a pulper, where it tends to go near the center of the vortex, and the same block can sometimes be seen circling there for half an hour.
Sooner or later, of course, the block of styrofoam is caught by the strong turbulence in the center of the vortex, and entrained in the downward part of the secondary circulation, until it is struck by the rotor. Unlike sheet and film plastic, the styrofoam is quite friable, and when struck by the rotor will disintegrate into a number of smaller pieces. The smaller pieces, of course, are more readily entrained by the flow, strike the rotor again, and break down to smaller and smaller pieces. The result is that styrofoam is broken down to pieces of small enough size to pass through the holes in the extractor plate with the stock leaving the pulper. It is usual to pass the pulped stock through screens, which will remove much of the broken down styrofoam, and then through reverse cleaners, which will remove still more. However, where very small particles of styrofoam are concerned, it must be appreciated that pulp screens, or centrifugal separating devices like reverse cleaners operate only on a sort of statistical probability basis, and cannot be guaranteed to remove every particle. When such particle reaches the paper machine, it is incorporated into the sheet. When it passes through the calender stack, the piece of styrofoam is compressed, but then springs back, resulting in a very undesirable raised defect in the sheet, often called a "pimple" in the sheet. On examination, it will be found to contain a small encapsulated piece of styrofoam.